In recent years, energy production has experienced a shift from petroleum towards coal as a source of thermal power. As a result, one technical problem which has arisen is the need to make turbine generators of increasing effectiveness. Because space is usually limited, the capacity of each individual generator tends to increase.
The rotor shafts of large electric generators are made of steel. Such shafts are very special objects. The shafts for the new generation of large thermal power plants, some of which are envisaged to output as much as 1,000 MW or more, may weigh of the order of 80 tonnes. They must withstand fast rotation, and yet remain operational for a period measured in decades.
Therefore, very high strength and very high toughness are needed. It is well known that high strength tends to cause low toughness, and vice versa. That is one problem. Furthermore, because of the use of the material, it needs to have suitable magnetic properties.